1. Field of the Invention
The present invention relates to a plunger-type electromagnetic actuator for use in an electronic governor of an internal combustion engine.
2. Description of the Prior Art
A plunger-type electro-magnetic actuator is adapted to function as an electric-mechanical converter to control the movement of a fuel control rod in a electronic governor of an internal combustion engine. conventionally, a spring-biased plunger is movably disposed in an electro-magnetic solenoid. The position of a fuel control rod connected to the plunger is determined by the resultant of the forces produced by the spring and by the exciting current provided to the solenoid winding. However, as shown in FIG. 1, the electro-magnetic attraction force F applied to the plunger due to the current flowing in the solenoid winding is approximately inversely proportional to the square of the distance X between the plunger and a magnetic pole piece, even if the exciting current provided to the solenoid winding is maintained constant. Due to this relationship, the exciting current should be accurately controlled when the distance X is small. However, such accurate current control is extremely difficult to achieve so as to control accurately the position of the fuel control rod throughout the range of travel. One approach to achieve the desired accuracy in positional control of the fuel control rod within a predetermined range is to locate the range in the region where the attracting force is at a low level, i.e., the gap length X is large. To do so, however, would make it difficult to provide a compact structure, and also the exciting current becomes large per unit of attraction force.